The preferential coupling of a magnetic field to ionized
material implies a two-layered structure in protoplanetary
accretion disks due to partial activation of the
magnetorotational instability. This structure provides a
disk instability that is capable of explaining the large
fluctuations of brightness in FU Orionis objects, which are
often thought to originate in a sudden increase of
protostellar disk accretion. In the current work, we
determine the importance of the active layer in these
phenomena by analyzing the ability of an averaged one-layer
model to reproduce outbursts with appropriate timescales and
magnitudes. W use the results to determine constraints on
protoplanetary disk structure.